400 Mr. S. H. Freeman on the Question 



consisted of an evaporating-dish, 19 centim. in diameter and 

 1 centim. deep, insulated, and connected with one pair of quad- 

 rants of a Thomson's quadrant-electrometer. The other pair 

 of quadrants and the case of the electrometer were connected 

 with earth. The evaporating-dish was supported on a wire 

 frame suspended by silk threads. To prevent disturbances 

 due to electricity on neighbouring objects, particularly the 

 clothes of the observer, it was found necessary to enclose the 

 whole apparatus in a metallic case. Openings were made in 

 this case, to facilitate evaporation and to permit the mirror of 

 the electrometer to be seen. The commutator was governed 

 by a silk thread passing through an opening in the case. In 

 the experiments the results of which are given in Table II., 

 the evaporating-dish was of copper ; in the other cases it 

 was of tin. The liquids examined were alcohol, sulphuric 

 ether, water, and solutions of NaCl and of CuS0 4 . The last 

 three were most carefully studied — water and NaCl because of 

 their importance in evaporation in nature, and CuS0 4 because 

 of the very large deflections which Tait and Wanklyn obtained 

 with it. 



The electrometer was about thirteen times as sensitive as 

 Prof. Tait's. Its deflections were reduced to absolute measure 

 by comparison with a Daniell's cell, the deflections produced 

 by which were observed before and after each series. The 

 capacity of the evaporating-dish and its connexions was found, 

 by comparison with Prof. Rowland's standard condenser, to 

 be 65 centim. nearly. The rate of evaporation was determined 

 by weighing the evaporating-dish and liquid before and after 

 each series. It was found impossible to eliminate every source 

 of electricity except evaporation, since deflections were always 

 obtained even with the evaporating-dish dry. In order to 

 correct for this, each series began and ended with two or more 

 observations of the deflections obtained by insulating for a 

 known time, usually five minutes, the dry evaporating-dish. 

 Between these observations, several similar observations with 

 the liquid evaporating were made. 



As the result of a large number of observations, in which 

 the evaporating-dish was insulated for five minutes, 



Alcohol gave a potential ) A . A/I , ,. , v ,. „ , . ^ . „, 

 from * . . . . j — 0*04- to -0-09 that of a Daniell's 



Cotton wet with alcohol 1 [cell, 



gave a potential from J + °" 02 to ~0"06 „ 



{Sulphuric ether gave a ") n ™ , 

 potential from. . j-0'02to- 



0-10 



